1. Field of the Invention
This invention relates to apparatus for reading permanently structured magnetic records having a first coercivity, comprising an oscillator having an inductive read head which is coupled to a circuit arrangement.
2. Description of Related Art
A known apparatus of this type is described in GB-A-2035659. This apparatus can read data from specially constructed magnetic tape consisting of aligned regions of acicular magnetic particles having a coercivity of the order of 300 oersteds (i.e. 24 kA/m). This magnetic tape is known by the trademark xe2x80x9cWATERMARKxe2x80x9d.
xe2x80x9cWatermarkxe2x80x9d tape, being made from xe2x80x9csoftxe2x80x9d magnetic material, can have non permanent data recorded on it in addition to this permanent structure. One of the features of the active head circuit described in GB-A-2035659 is that it erases any xe2x80x9csoftxe2x80x9d data which might have been recorded on the tape. This feature is useful to prevent fraud using non-permanent recorded data. The acicular magnetic particles used in this tape are made from low coercivity (LoCo) material.
In recent years there has been a growth in the use of high coercivity (HiCo) magnetic records, for example as tape on credit or bank cards or other documents of value. Such material typically has a coercivity of above about 1200 oersteds (i.e. 100 kA/m) and is described more fully in the standards document ISO 7811/6. The known read head described in GB-A-2035659 does not have a powerful enough magnetic field to erase data recorded on such high coercivity tape. However, the inventor has found that such high coercivity tape cannot be used fraudulently to counterfeit xe2x80x9cWatermarkxe2x80x9d tape as the known read apparatus does not respond to erasable data on high coercivity magnetic tape. Although this is an advantage for the security of xe2x80x9cWatermarkxe2x80x9d tape systems, it is a disadvantage for a general purpose magnetic reader apparatus because two different read heads or two separate read operations would be required if it was desired to read both types of data using the same apparatus.
According to the present invention there is provided apparatus for reading permanently structured magnetic records having a first coercivity, comprising an oscillator having an inductive read head which is coupled to a circuit arrangement, characterised in that at least one side of the read head sees an impedance which is larger than the impedance of the read head over the range of frequencies of interest such that the read head provides an electrical signal in response to magnetic records having a second coercivity which is higher than the first coercivity. This apparatus enables a single read head to be used to read both xe2x80x9cWatermarkxe2x80x9d data and high coercivity data. The high coercivity data may be permanently structured or erasable.
Preferably, the electrical signal is derived from the voltage across the read head. This voltage is advantageously captured by a differential integrator which subtracts the voltages on either side of the read head to give a voltage difference, and integrates this difference over time.
The range of frequencies of interest is conveniently lower than the frequency of operation of the oscillator. Preferably lower than 10 kHz, very preferably lower than 5 kHz.
The invention relates particularly, though not exclusively, to high security magnetic data reader systems.